1. Field of the Invention
The present disclosure relates to a dichalcogenide thermoelectric material having a very low thermal conductivity in comparison with a conventional metal or semiconductor.
2. Description of the Related Art
In general, thermoelectric materials can be utilized in active cooling, waste heat power generation, and the like by using the Peltier effect and the Seebeck effect. FIG. 1 is a schematic diagram illustrating thermoelectric cooling using the Peltier effect. Referring to FIG. 1, the Peltier effect occurs when a DC voltage is applied and holes of a p-type material and electrons of an n-type material are transported, causing heat generation and heat absorption on both ends of the p-type and n-type materials. FIG. 2 is a schematic diagram illustrating thermoelectric power generation by the Seebeck effect. Referring to FIG. 2, the Seebeck effect occurs when heat is supplied from an external heat source and a flow of a current is generated in the material while electrons and holes are transported, resulting in power generation.
Active cooling with these thermoelectric materials improves the thermal stability of devices, does not cause vibration and noise, and does not use a separate condenser and refrigerant. Therefore, the volume of these devices is small and the active cooling method is environmentally friendly. Thus, active cooling that uses such thermoelectric materials can be applied in refrigerant-free refrigerators, air conditioners, microcooling systems, and the like. In particular, when a thermoelectric device is attached to a memory device, the temperature of the device can be maintained in a uniform and stable state, as compared to conventional cooling methods. Thus, the memory devices can have improved performance.
In addition, when thermoelectric materials are used in thermoelectric power generation using the Seebeck effect, waste heat can be used as an energy source. Thus, thermoelectric materials can be applied in a variety of fields that increase energy efficiency or reuse waste heat, such as in vehicle engines and air exhausts, waste incinerators, waste heat in iron mills, power sources of medical devices in the human body powered using human body heat, and the like.
As a factor for determining the performance of such thermoelectric materials, a dimensionless figure-of-merit ZT defined as Equation 1 below is used.
                    ZT        =                                            S              2                        ⁢            σ            ⁢                                                  ⁢            T                    k                                    Equation        ⁢                                  ⁢        1            wherein
S is a Seebeck coefficient,
σ is electrical conductivity,
T is absolute temperature, and
κ is thermal conductivity.
To increase the performance of such thermoelectric materials, the values of the dimensionless figure-of-merit ZT should increase. Accordingly, there is a need to develop a material having a high Seebeck coefficient and electrical conductivity and low thermal conductivity.
Many kinds of thermoelectric materials have been developed. However, many thermoelectric materials perform well only in a range of room temperature to high temperature. For example, Bi2Te3 and a solid solution compound thereof are known thermoelectric materials having high performance at about room temperature (300 to 400 degrees Kelvin (“K”)).
However, there is a need to develop a variety of thermoelectric materials that perform well over broader temperature ranges. For this, there is an increasing interest in thermoelectric materials having a dichalcogenide structure.
For example, U.S. Patent Publication No. US2003/0056819 and Japanese Patent Laid-Open Publication No. P2002-270907 by NEC, Japan disclose a conventional dichalcogenide thermoelectric material having a two-dimensional layered structure. The thermoelectric material is represented by Formula AxBC2-y where 0≦x≦2 and 0≦y<1, wherein A comprises at least one element selected from the group consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, Au, Sc, Y, and a rare earth element, B comprises at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Ir, and Sn, and C comprises one of S, Se, and Te. In Examples disclosed in these NEC applications, thermoelectric characteristics of an AxTiS2 material are described, and ZT values are reported to be very high, i.e., 2.9 at room temperature, 3.9 at 700 K, and the like. However, there appear to be no references that verify these ZT values in any subsequently reported materials, and in fact, ZT values of AxTiS2 are reported to be no more than 0.2 at room temperature (see Phys. Rev. B, vol. 64, 241104, 2001 and J. Appl. Phys., vol. 102, 073703, 2007). Accordingly, the thermoelectric material disclosed in this application is not in wide use.
In addition, in 2007, Catalin Chiritescu et al. manufactured a WSe2 thin film having a very low thermal conductivity (see Science, vol. 315, p. 351, 2007). WSe2 having a two-dimensional layered structure can have very low thermal conductivity, i.e., about 0.05 Watts per meter-Kelvin (“Wm−1K−1”) when thin films are stacked irregularly in an in-plane direction and regularly in a c-axis direction. This means that materials having a 2-dimensional disordered and layered structure within the in-plane direction, but regularly stacked in the c-axis direction can have a very low thermal conductivity. However, such thermoelectric materials, which are insulators, have very low electrical conductivity, and thus are unsuitable for use as a thermoelectric material. In addition, it is difficult to make materials with random arrangement in an in-plane direction in bulk.